RF receivers often use correlators to correlate a code in a received RF signal with a reference code. Once the received code is correlated with the reference code, other information in the received signal may be extracted. The other information may be used in applications associated with equipment containing the RF receiver, such as a Global Positioning System (GPS) receiver, a navigation system receiver, or the like. In some applications, the reference code may be a frequency spreading code that is used in a direct sequence spread spectrum (DSSS) system. In a DSSS system, an RF transmitter sends data using a modulated RF signal. A bit sequence having a higher rate than a data rate may incorporate the frequency spreading code to spread the frequency content of the modulated RF signal, and may incorporate redundancy in the transmitted data.
Some RF receivers may receive desired RF signals in the presence of interfering signals, which may be continuous wave (cw) interfering signals, modulated interfering signals, or interfering signals having embedded correlation codes. Some signals, such as GPS signals from satellites, may have Doppler frequency shifts. In a GPS system, a GPS receiver may be receiving desired RF signals from some GPS satellites in the presence of stronger RF signals from interfering satellites. Therefore, filtering may be required to remove the effects of interfering signals. Thus, there is a need to both filter RF signals to remove interference and to correlate a reference code with a desired RF signal.